A field study in progress of the sociobiology and ecology of lion-tailed monkeys (Macaca silenus) is yielding data on socioecological correlates among group size, activity rates, movements, spacing, patterns of habitat exploitation, and fluctuating environmental factors. Companion field studies of the sympatric Nilgiri langur (Presbytis johnii) and bonnet macaque (Macaca radiata) have been initiated in the same rain-forest habitat using the same methods of recording intensive and systematic data. Quantitative and statistical comparison of the ethological and ecological results on these three species will refine our understanding of the role of environmental agents, particularly resource distribution, on the determination of social behavior. Comparison with other primates can help elucidate the magnitude of different evolutionary forces on sociality. Study of evolution of social behavior in primates must consider kinship relations within a troop and how these affect transmission of behaviors which confer survival advantages. Lion-tailed macaques share foods, learn from each other what foods to eat and how to eat them, and vocalize when they find foods. Long-term observations are being conducted on two troops of unprovisioned and individually-known lion-tailed monkeys. This study thus meets three requirements crucial for interpreting the interaction of behavior with social evolution: undisturbed animals with known kinships, frequent food-sharing and social learning behaviors, and a vocal repertoire rich in sounds used when food is located. The vocal repertoire of M. silenus is being analyzed from field tape recordings and compared with that of the congeneric Japanese monkeys whose vocalizations are known. The social usage of the variables vocal patterns of M. silenus will be analyzed and described. New field observations are concentrating on the use of those sounds related to localizing food resources, an aspect of vocal communication.